In battery-powered cameras there have long been known a variety of range sensing systems for automatically setting the focus of the lens to correspond to the distance of an object centrally located in the field of view, or alternatively for adjusting the aperture in flash mode where the aperture setting is also a range-dependent parameter. Such systems have employed ultrasonic or infra-red signals directed at the object of interest. The infra-red signal sources were generally gas filled flash lamps. In some cases, the same flash lamp used for illuminating the subject is also used in a pre-flash mode to first generate the range determining signal. In other cameras, a separate gas flash lamp is used for range determining purposes. In either case, the distance of the object from the camera is determined by measuring the amplitude of the reflected signal. The use of gas filled flash lamps as the source of infra-red energy is relatively costly as compared, for example, to the cost of using infra-red light emitting diodes; however, the intensity of the energy directed by such diode sources has heretofore been of such low intensity that it is believed that only a triangulation means using relatively expensive circuitry for determining range was thought to be practical.
An example of a ranging system which uses a triangulation based measuring system is currently marketed by Hamamatsu Photonics of Hamamatsu City, Japan, and is described in their technical data sheet entitled "Autofocus Devices" dated March, 1984. The system described therein is based upon an optical triangulation system wherein a light-emitting diode is powered at relatively low power and focused into a beam directed at an object centrally located in the scene to be photographed. A position sensing diode is disposed to receive reflected light focused thereon by a lens, the position of the spot on the diode being used to derive a difference signal used to govern signal processing circuitry so that the output signal used to govern the relevant camera parameter is related to the range of the object.
The photosensing diode is a three-terminal device in the form of a P-I-N diode having a common electrode on the rear face thereof and a pair of output electrodes disposed on the light-receiving face on either side of a central sensing region. The position sensing diode is operated in biased photovoltaic mode, and appears to provide a voltage difference at the two output terminals according to the position of the incident light spot with respect to the center of symmetry of the structure. The signal processing circuitry associated with such a device appears to be quite complex, involving a pair of signal processing circuits. Each signal processing circuit in turn feeds a distance calculation circuit, the output of which is fed to a sample-and-hold or alternatively to an analog-to-digital converter to govern the adjustment of the relevant camera parameter. The circuitry necessary to accomplish this is quite expensive, and the photosensing diode itself, being a three-terminal device, is expensive because of the number of terminals alone, as well as because of the highly specialized nature of the device itself. Additionally, the light emitting diode appears to be driven at a relatively low power level of 10 milliwatts, which would raise a severe signal-to-noise problem. This in all likelihood requires a continuous modulation of the diode driver power, coupled with some form of synchronous detection in the signal processing circuitry; however, no such feature is shown explicitly in the aforementioned Data Sheet.
There remains a need in the case of cameras for the intermediate price market for an inexpensive infra-red range sensing system which does not have the complexity, and hence the expense, of the systems of the type described hereinabove. Moreover, as applied to cameras having two-position lenses wherein the lens is set to either a far-focus position placing the distant edge of the field at infinity (the hyperfocal distance) or to a near-focus setting substantially closer than the hyperfocal setting, a simple range finding system capable of providing reliable ranging information for objects out to no more than 15 feet or so would be more than adequate, provided that it can be accomplished at minimum cost.
To the applicant's knowledge, no such system has yet been provided.